The present application relates to the field of imaging, and in particular imaging modalities that produce images utilizing radiation technology (e.g., at times referred to herein as radiography imaging modalities). It finds particular application with medical, security, and/or other applications where obtaining information about physical properties (e.g., density and/or effective atomic number, etc.) of an object under examination may be useful.
CT and other radiography imaging modalities (e.g., single-photon emission computed tomography (SPECT), mammography, digital radiography, etc.) are useful to provide information, or images, of interior aspects of an object under examination. Generally, the object is exposed to radiation photons (e.g., such as X-rays, gamma rays, etc.), and an image(s) is formed based upon the radiation absorbed and/or attenuated by the interior aspects of the object, or rather an amount of photons that is able to pass through the object. Traditionally, the image(s) that is formed from the radiation exposure is a density image or attenuation image, meaning the image is colored/shaded as a function of the respective densities of sub-objects comprised within the object under examination. For example, highly dense sub-objects absorb and/or attenuate more radiation than less dense sub-objects, and thus a sub-object having a higher density, such as a bone or metal, for example, will be shaded differently than less dense sub-objects, such as muscle or clothing.
While such imaging systems have proven successful, in some applications imaging based upon more than density may be advantageous. For example, in security applications, certain threat items can be hidden amongst clothing or other non-threat items that have similar densities to the threat items. Thus, in some applications, such as airport security, it may be useful to determine other and/or additional physical properties of the item under examination, such as, for example, effective atomic number (e.g., at times also referred to as z-effective). In this way, threat items that have densities similar to non-threat items, but different atomic numbers, for example, can be more correctly identified.
As early as 1992, multi-energy imaging modalities have been deployed in some environments, such as airport and military establishments, to provide additional information about an object under examination, such as an object's effective atomic number(s). These modalities have provided improved detection and false alarm performance when tasked to differentiate between sub-objects, relative to systems that merely differentiate sub-objects based upon density.
Multi-energy imaging modalities operate by using multiple, distinct radiation photon energy spectra to reconstruct an image(s) of an object. Such distinct energy spectra can be measured using numerous techniques. For example, multi-energy measurements can be performed using energy resolving detectors (e.g., where the detectors are configured to selectively detect radiation having an energy within a first energy spectrum from radiation having an energy within a second energy spectrum), two radiation sources (e.g., respectively emitting radiation at a different energy level/spectrum), and/or by varying the voltage applied to a radiation source (e.g., such that the energy of emitted radiation varies as the applied voltage varies).
Two widely-used multi-energy imaging modalities are a dual-energy CT scanner and a dual-energy line scanner (e.g., also referred to as a dual-energy projection scanner). While both have proven effective, respective scanners have drawbacks. For example, dual-energy CT scanners are costly to produce because they generally require detectors that are more costly to manufacture than detectors used on single-energy scanners and/or require one or more high voltage sources that can rapidly modulate the power supplied to a radiation source(s). Dual-energy line scanners, while less expensive to manufacture than dual-energy CT scanners, generally have a higher false alarm rate than dual-energy CT scanners (and thus may be unable to pass security certifications) because dual-energy line scanners typically yield data from, at most, a few views, for example. Thus, the more sub-objects within an object (e.g., the more densely compacted items are in a suitcase), the more difficult it is for a dual-energy line scanner (e.g., which generally merely generates a top-down image) to discriminate between the density and/or atomic number of a first item and a second item if the items are stacked on top of one another, for example. Stated differently, the more crowded the suitcase, the less likely it is that a dual-energy line scanner will have a clear line integral measurement from which to derive a z-effective for a particular sub-object of interest.